This invention pertains to valve position control loops in steam turbine control systems, and particularly to an adjustable gain controller as part of the digitized portion of the valve position control loop. A method for reducing the jitter in the control loop is also described herein.
The incorporation of digital components into the control system of a steam turbine is advantangeous for many reasons widely known to those of ordinary skill in the art. In the past, some steam turbines have been controlled by analog systems which utilize analog electrical signals, and hydraulic signals to control the mechanical devices. One recognized turbine control system which is completely analog in nature is described in U.S. Pat. No. 3,097,488, issued to Eggenberger et al., and that disclosure is incorporated herein by reference thereto. Generally, the Eggenberger et al., patent describes a series of analog electrohydraulic control loops which interact with each other to regulate steam flow into the turbine and thereby control the speed and output torque of the turbine.
In a number of steam turbine control systems, the basic principles of an analog electrohydraulic control system as described in the Eggenberger et al., patent are utilized in addition to a digital control system. U.S. Pat. No. 3,709,626, issued to Eggenberger discloses a digital/analog electrohydraulic turbine control system. The disclosure in that patent is incorporated herein by reference thereto.
A computer based control system for the turbine-generator plant is described in U.S. Pat. No. 4,280,060, issued to Kure-Jensen et al. and the disclosure of that patent is incorporated herein by reference thereto.
Although the incorporation of computer based, digital control systems into the operation of a steam turbine has many advantages, the valves controlled may be analog valves, and some portions of the control loop may remain analog to interface with those valves. Specifically, while parts of the valve position control loop may be digitized, i.e., capable of handling digital signals therein, the remaining portions may be analog in nature. In particular, the control valve positioning subloop (sometimes referred to herein as the electrohydraulic valve actuator) and the bypass valve positioning subloop are usually completely analog in their operation and structure.
The analog valve positioning subloops include a steam valve which is opened and closed by great mechanical forces applied to its valve stem. The force on a typical bypass steam valve is on the order of 10,000 to 20,000 pounds. These forces are developed by an hydraulic actuator which is mechanically attached to the valve stem. High pressure hydraulic fluid is supplied to and vented from the actuator through a servovalve. The servovalve converts analog electrical signals input thereto into hydraulic signals. Somewhere in the subloop, there may be a non-linear function generator which corrects the analog electrical signal to provide for a linear steam flow through the valve which is well known in the art. The subloop is supplied with an analog electrical signal by the primary valve position control loop. Additionally, a feedback signal from the steam valve is fed to a summer which sums the analog electrical signal from the primary control loop with the feedback signal. The summed output ultimately is applied to the electrical input of the servovalve thereby completing the valve positioning subloop. A better understanding of the valve positioning subloop can be obtained from the aforementioned U.S. Pat. No. 3,097,488.
Since the valve, valve hydraulics, valve position and process sensors are analog in nature, when the valve control loop operates in conjunction with a computer based control system the inputs and outputs of the computer must be converted from analog to digital and digital to analog. As is well known by those skilled in the art, the conversion is accomplished with A to D converters and D to A converters (hereinafter A/D or D/A converters). One of the problems which arises with the partially digitized control system is the inability of the A/D converter to provide a completely accurate digital signal for all analog signals input thereto. These problems relate to the quantization of the analog signal and the sampling time of the A/D converters. The problems are aggravated by the noise inherent in any control loop and particularly in the primary valve positioning control loop.
As is well known in the art, it is common to include within the valve position control loop a gain amplifier which provides for the regulation of the steam valve being controlled. The regulation of the steam valve is the instantaneous percent change in the error signal which would cause the demand for the steam valves to instantaneously change between wide open and closed. For example, if the control system normally recognizes a 0 digital signal for a control signal of zero, and a 4096 digital signal for a maximum control signal and the operators establish a regulation value for that particular valve at five percent, the steam valve would be demanded to go through full stroke when the digital control signal varies instantaneously 205 digital levels from the set point of the control signal. As is well known in the art, the set point for the control signal is a preselected signal established by the turbine operators. The valve position control loop generates an error signal between the set point and the feedback signal from a sensor responding to steam flow conditions. The aforementioned U.S. Pat. No. 3,097,488 clearly explains the regulation of steam valves. A five percent regulation for a bypass steam valve is relatively common in the industry. Also, the valve position control loop must respond to the turbine control system signals relatively quickly. The time in which the bypass steam valve must open is in the range of 2-4 seconds.
As is well recognized in the art, the overall gain of the primary valve position control loop, when a relatively low regulation is established, is relatively high because the gain varies inversely with the regulation value. In other words, if the regulation of the controlled valve is set at five percent, the overall gain of the valve position control loop will be twenty, the reciprocal of the regulation value. Small digital signals, generated either by the turbine control system or by the feedback in the valve position control loop, are amplified twenty times and the amplified signals are applied to the valve positioning subloop which effects the positioning of the valve.
The parameter being controlled, typically steam flowing through the valve, is not substantially influenced by high frequency digital signals even though the signals are amplified twenty times by the valve position control loop. However, the servovalve, the hydraulic system associated with the servovalve, the steam valve actuator, the mechanical connections between the actuator and the steam valve stem, and all other mechanical connections attached to the servovalve and associated hydraulic system experience a pulsing action or jitter due to these amplified digital signals. The jitter or pulsating action may cause premature or shaking wearing of the servovalve, the hydraulic system associated therewith, and/or the valve actuator. A better understanding of the specifics of this problem can be found in the illustrations herein and their corresponding description.